In a device such as a laser printer, a projector, or an image scanner, a two-dimensional scanning optical device for scanning a light beam irradiated from a light source on a surface to be scanned is mounted. Such type of a scanning optical system of the two-dimensional scanning optical device has an f-θ characteristic, and scans the light beam in a main scanning direction at a substantially constant speed without an unevenness in an intensity of light. Further, a scanning speed of the light beam in the auxiliary scanning direction is also substantially constant. Thus, intervals between scanning lines scanned on the surface to be scanned are kept at substantially equal intervals. On the other hand, in the two-dimensional scanning optical device, a problem arises such that the scanning lines are curved.
In order to overcome such a problem, a two-dimensional scanning optical device including a scanning optical system having an f-sin θ characteristic has been proposed, for example, in Japanese Patent Provisional Publication No. HEI01-163717A. When a scanning optical system is an optical system having the f-sin θ characteristic, the problem, in which the scanning lines are curved, does not arise.
However, when a scanning optical system of a two-dimensional scanning optical system has the f-sin θ characteristic, another problem arises in which intervals between scanning lines becomes narrower as they apart from a center of a scanning area on the surface to be scanned, since the f-sin θ characteristic has a negative distortion. In general, such a problem can be overcome by performing an electrical correction such as an image interpolation process or a driving speed control of a deflector.
However, intervals between scanning lines are not ultimately (optically) corrected by a correction based on the image interpolation process. Thus, a problem on an unevenness of light intensity or degradation of an image remains. Further, for a correction based on the driving speed control of the deflector, a driving system of a deflector becomes complicated and a processing load (processing time) of the driving control system increases. Thus, a problem arises in that, for example, a frame rate has to be reduced. Therefore, it is not preferable to correct the intervals between the scanning lines through an electrical process. In order to optically correct the intervals between the scanning lines, it can be considered to configure the scanning optical system with an anamorphic optical system, as disclosed, for example, in Japanese Patent Provisional Publication No. 2004-138748A. However, the anamorphic optical system has a shape which is rotationally asymmetric with respect to an optical axis. Therefore, a metal mold for the anamorphic optical system cannot be easily produced using a lathe. A special device such as a three-dimensional processing device is required for producing the metal mold. Thus, there is a problem such that a production cost increases.